Make the character-limited textarea more intuitive

[ConnorKrammer]

This solves the problem raised in issue #5. Namely, it:

• Prevents input from exceeding the textarea limit when holding down a key
• Prevents pasting text that would exceed the textarea limit
• Updates the progress bar even when holding down a key

The solution is cross-browser, though very old browsers might not work with it.
This should also fix issue #1.

Review welcome.

[afshinm]

That's a ton of changes @ConnorKrammer! Could you please provide an online example?

[ConnorKrammer]

I just looked back on this and realized I could have done this much more simply, so I've updated it accordingly. That's cut down on the changes by a lot.

If you want to see the changes in action, open up my fork's demo page. Keep in mind that I made no changes to progress.js itself, just the demo.

Let me know if there's anything to fix.